Annoying exhaust rattle/vibration

My '00 SE (with 60K on the odometer) has developed a very annoying exhaust rattle/high pitched vibration noise at around 1500-2000 RPM and *only* after the engine wams up and *only* while driving. First I checked all the heat shields and baffles, nothing loose, all tight and very little rust. I even took it in but they couldn't find anything. They suggested it may be coming from inside the cat which is scary. Anyone experience this?

They may seem tight, but it's probably a shield around the pipe. Mine did the same thing and it seemed fine until I actually started removing the shields. That's when I found a broken shield mounting tab and some clearance issues. Those shields are pathetic and they mount metal to metal on the exhaust, so it's actually surprising to me that they don't make more noise than they do.

You could also have a rock stuck between the pipe and the shield.

I removed all of that wrap around shielding.